Tunable lasers are widely used in the field of optical communication. Especially with the development of high speed optical communication network in recent years, a demand for tunable narrow linewidth lasers is continuously growing. The tunable lasers will play an important role in the future optical communication network. According to structure, the tunable lasers may be classified as monolithic integrated tunable lasers and external cavity tunable lasers. The monolithic integrated tunable lasers have advantages such as small size and good stability, but at present they have a linewidth larger than that of the external cavity tunable lasers. Thus, the monolithic integrated tunable lasers are not suitable for the development of high speed optical communication network. The external cavity tunable lasers have advantages such as narrow linewidth, large tuning range and less technical challenges, and they have taken a large share in the current commercial 100G optical communication system. Compared with the monolithic integrated lasers, however, the external cavity tunable lasers have worse stability and are more likely to experience mode hopping under the influence of external interference, which degrades characteristics of the laser and thus performance of the optical transmission system. In order to utilize the external cavity tunable lasers, a critical task is to suppress mode hopping thereof. In this regard, monitoring and online-adjusting conditions of the external cavity tunable lasers are very important.
A lot of solutions have been proposed for the external cavity tunable lasers of various structures to suppress mode hopping and accomplish wavelength locking. Generally, such solutions relate to monitoring and compensating for amplitude or phase deviation. A commonly used solution is to determine the output frequency of longitudinal mode by the maximum power algorithm or the special local slope algorithm for optical power spectrum and finely adjust the longitudinal mode output frequency by a phase tuning element in the external cavity, so as to lock the cavity mode. However, to maintain the long term stability of the tunable laser, the phase tuning element should have quick response and a large phase tuning range. Usually, one phase tuning element cannot meet the two requirements at the same time. Therefore, the best choice is to adopt a combination of a phase tuning element with quick response and a phase tuning element with a large phase tuning range.
The external cavity tunable lasers have an obvious drawback of being likely to experience mode hopping under the influence of external interference, which may degrade characteristics of the lasers. Suppressing mode hopping is a sophisticated technology that has to be solved to make use of the external cavity tunable lasers. There have been various solutions proposed for the external cavity tunable lasers with different structures to suppress mode hopping and accomplish mode locking. In general, such solutions relate to monitoring and compensating for amplitude or phase deviation. Therefore, monitoring and online adjustment of conditions of the external cavity tunable lasers are very important.